In traditional anti-lock braking systems (ABS) and electronic stabilisation programs (ESP), hydraulic control valve assemblies are used, sometimes referred to as an ABS hydraulic modulator unit. Hydraulic control valve assemblies typically contain valves, solenoids, and pistons to control the holding and release of the different hydraulic brake circuits under hard braking situations of vehicles. During normal operation the system lies dormant and the standard brake system is used. When a hard braking situation occurs, the system senses the rotational speed of the wheel speed sensors and makes decisions as to whether to hold or release pressure to a brake circuit. In traditional ABS and ESP, it is known that at least some of the valve coils may no longer be switched. Instead, analogised actuation is effected using pulse width modulated current control (PWM), which permits an almost analog actuation of the hydraulic valves.
Generally, a plurality of valve actuation circuits are provided, which are typically designed using metal oxide semiconductor (MOS) transistors connected in phase-opposition. These valve actuation circuits are usually implemented as integrated circuits, especially as some complex ABS systems require up to eight such valve actuation circuits to be present and complex ESP systems require up to twelve such valve actuation circuits to be present, in addition to numerous additional circuit parts.
Referring to FIG. 1, a block diagram of part of a known PWM current sensing application 100 is illustrated comprising, a first valve actuation circuit 102 and a second valve actuation circuit 104. The first valve actuation circuit 102 comprises a high-side current sense amplifier 106 and a low-side current sense amplifier 108, wherein the high-side current sense amplifier 106 and low-side current sense amplifier 108 both comprise an analog-to-digital converter (ADC) and a sigma delta modulator. The second valve actuation circuit 104 also comprises a similar high-side current sense amplifier 106 and a low-side current sense amplifier 108.
Therefore, in the known PWM current sensing application 100, each valve actuation circuit 102, 104 requires two sense amplifiers 106, 108. Thus, a typical ESP application that requires at least eight activation circuits would require at least sixteen current sense amplifiers.
Referring to FIG. 2, a known example of a current measurement circuit 200, used in the switch-on path of a valve actuation circuit of FIG. 1, is illustrated. The known example of a current measurement circuit 200 comprises, a load current iL 201, which represents the inductive behaviour of a valve coil, and the channel resistance of a power driver 207 to ground when the power driver 207 in the switch-on path is enabled. The load current iL 201 is established on the basis of a voltage Uref 203 measured across inductance L 205.
In this known example, a summing junction 209, integrator 211, comparator 213 and 1-bit digital-to-analog (DAC) 215 comprise a sigma delta modulator. The summing point 209 outputs a combined signal to the integrator 211, which is used as a controller and whose output forms the input potential for the comparator 213 relative to ground. The comparator 213 is part of the control path of the control loop and is operated in clocked fashion. The output of the control loop is fed back via the 1-bit DAC 215.
In this way, the 1-bit DAC 215 is in the form of a switchable current source 217, which drives a defined reference current through a sense FET 219 to ground. The gate connections of the power driver 207 and the sense FET 219 are connected to one another, resulting in these two transistors being actuated together when the control loop is in operation and only when the power driver 207 is enabled.
The sigma delta modulation is performed at a frequency that is 256 times as high as the frequency of the PWM, corresponding to a 256 fold oversampling.
Currently, known current sensing systems for PWM drivers use a separate sense amplifier and ADC sigma delta converter for each of the high and low-side of a valve actuation circuit. This is wasteful of current consumption and utilises unnecessary die space.